In recent years, with rapid spread of small mobile terminals represented by smart phones and tablets, a demand for small batteries having high energy density, which drive the small mobile terminals, is increasing.
Currently, a graphite-based material is used for negative electrodes of many lithium ion batteries. A theoretical capacity of the graphite-based material is 372 mAh/g (LiC6), and currently, the graphite-based material is used near the limit. Since a negative electrode material is a reducing agent, a material having a potential as low as possible, strong reducing power, and small electrochemical equivalent is favorable. Therefore, elements alloyed with lithium, such as silicon and tin, which have a second lowest potential after carbon and lithium, and high capacity density, amorphous chalcogen compounds, and the like have drawn attention as the negative electrode materials for next-generation lithium ion battery.
Among them, silicon can store lithium atoms up to the ratio of 4.4 to one silicon atom, and can theoretically have the capacity about ten times that of the graphite-based carbon. However, there are problem that, when a silicon particle stores lithium, its volume is increased to about three to four times, and especially, when the particle size is large, the particle is cracked and pulverized. Meanwhile, it is known that, when the size of the silicon particle is nanosized, adjacent particles are united and cause grain growth with passage of a charge/discharge cycle, and cyclability is considerably decreased. Therefore, typically, measures of coating the surface of the silicon particle with a carbonaceous material, and the like are performed. However, this coating is coating a solid with a solid, and thus adhesive properties are not sufficient. A method of coating the periphery of a silicon particle with silicon carbide that has good bonding properties with silicon is known. While a silicon carbide coating layer suppresses volume expansion, and improves the cyclability, the silicon particles are made by pulverization, and the size is relatively large and the ratio occupied by silicon carbide in the coated particles is large. If the surface of the silicon particle is coated with the silicon carbide layer in a thick manner, the coating layer hinders movement of lithium and may impair an electrochemical reaction. Accordingly, there is a problem that sufficient charge/discharge characteristics cannot be obtained.